1. Field of the Invention
The present invention relates to a semiconductor device including a bipolar transistor formed on an insulating substrate.
2. Description of the Related Art
A so-called bipolar transistor formed on a wafer having an SOI (Silicon or Semiconductor On Insulator) structure is known as a bipolar transistor which has specific characteristics, e.g., a low parasitic capacitance and a low parasitic resistance and which is thus effective for realization of a low power consumption. Conventionally, in a lateral transistor of this type, a base layer is formed by implanting impurity ions in a semiconductor layer and annealing the resultant structure, or by thermally diffusing an impurity in a semiconductor layer. Also, a technique for forming a base layer by diffusing an impurity from a solid film containing the impurity into a semiconductor layer is also developed.
However, in any of the above methods, the formed base layer has a width larger than that of a vertical bipolar transistor. Also, in these base layers, the equi-concentration lines of the impurity concentrations form curves, as shown in FIG. 20B, due to the forming methods. In other words, the thickness of the base layer in a direction perpendicular to the pn junction becomes non-uniform, causing a difference in characteristics of the transistor, a decrease in breakdown voltage, and the like.